Map Multiple Objects and then Join into One Object with One UV Map?
Unless I am misunderstanding, you are just trying to preserve the UV mapping of each model while joining them to a single mesh?

If that is your goal, ctrl+J with both models selected in Object mode will do that.  The join will preserve the models UV maps as is.  It might get a bit weird if the two objects share the same material but the UVs will not have changed (though, they will be stacked on each other, but again, if the materials are assigned correctly it won't matter).